This invention relates, in general, to protection circuits, and more particularly to overvoltage protection circuits.
It is well known that integrated circuits have specific voltage requirements which cannot be exceeded. When the requirement is exceeded, it is known as an overvoltage condition. A power supply voltage which is larger than a specified maximum voltage can terminally damage the integrated circuit. The maximum allowable voltage is a function of the semiconductor process flow and to a lesser extent the type of circuitry on the integrated circuit.
The length of time the integrated circuit is exposed to an overvoltage and the magnitude of the overvoltage are the major factors which determine the amount of damage sustained by the integrated circuit in an overvoltage condition. In some cases, the overvoltage condition could produce a hazardous condition. For example, in a circuit that controls mechanical equipment being operated by a human being, the circuit damaged by overvoltage may send out erroneous signals that radically change operating characteristics of the mechanical equipment potentially placing the human being in a dangerous situation.
A specific situation where overvoltage protection circuits are needed is in an automobile. Integrated circuit technology pervades every aspect of the automobile from seat control to engine management. The reliance on integrated circuits has greatly enhanced the overall quality and efficiency of today's automobile but also poses a minute risk. The battery voltage of an automobile exceeds the maximum voltage of many commonly used integrated circuits. Wiring harnesses and other componentry can produce shorts which could expose an integrated circuit to an overvoltage. For example, an overvoltage condition to an automotive microprocessor may cause erroneous signals to be sent to a fuel throttle motor driver circuit that could lead to a dangerous situation. These types of situations could be prevented by decoupling the circuitry from the overvoltage condition.
External componentry to protect integrated circuits from an overvoltage condition are not cost efficient nor reliable enough. Ideally, the overvoltage protection circuitry is integrated on the integrated circuit itself. A critical aspect of any overvoltage protection circuit is that it can be fully tested to insure it will function correctly should an overvoltage condition occur.
Zener diodes and external fuses are commonly used to prevent overvoltage on an integrated circuit. The zener diode is easily integrated on the integrated circuit. A zener diode prevents overvoltage on the integrated circuit by breaking down above a predetermined voltage thereby limiting the voltage on the integrated circuit to the breakdown voltage. The zener diode does not provide adequate protection if the power source creating the overvoltage condition is low impedance. The power source will cause the zener diode to dissipate too much power and eventually self-destruct leaving the circuitry unprotected.
An external fuse protects the integrated circuit by decoupling the integrated circuit from the power source providing the overvoltage when current to the integrated circuit exceeds the rated current of the external fuse. The external fuse works in theory, but in practice the external fuse has been found to provide inadequate protection. External fuses are imprecise and very slow to open up. Furthermore, external fuses sense current not voltage. An overvoltage condition can occur that damages circuitry without opening the fuse. It is critical for many systems that an overvoltage condition be detected and circuitry shut down in a very short period of time. Furthermore, the external fuse adds additional components and increases manufacturing costs.
It would be of great benefit to provide an overvoltage protection circuit that is easily integrated on an integrated circuit, rapidly disables circuitry from receiving or sending erroneous signals, and is fully testable.